1. Field of the Invention
The invention is directed to a method of improving stability and performance of a four wheel drive vehicle, specifically through feed forward control that includes shifting torque from front to rear axles when the vehicle encounters a hill having a road surface with a low coefficient of friction.
2. Description of the Related Art
Typical four wheel drive vehicles now include means for shifting drive torque between the front and rear axles of the vehicle. Shifting is required in response to changing road conditions and/or driver requests transmitted via the accelerator pedal or steering wheel. When driving upwards on an incline, such as a hill, the weight of the vehicle rests in a greater percentage on the rear axle. Correspondingly, more drive torque should be shifted to the rear axle to maintain superior vehicle performance and handling. This distribution favoring the rear axle is especially important when the road on which the vehicle is traveling has a low coefficient of friction. Low friction situations are caused by snow, ice, mud, etc. Increased application of drive torque is a significant factor that minimizes slipping caused by low friction situations.
Shifting from a torque distribution ratio favoring the front axle or an even distribution, both of which are common when driving on a generally level surface, must be instigated by sensing changed road conditions. Typically, changed road conditions are sensed by recognizing slippage of the vehicle wheels. For example, when a vehicle traveling on a level road, with a torque distribution favoring the front axle, reaches a snow covered hill and begins to drive upward, slippage of the front vehicle wheels is first sensed. The slip condition is fed into an algorithm in the vehicle's onboard computer and resulting instructions shift torque from the front axle to the rear axle of the vehicle.
As a result of the procedure described above, the vehicle must be on a hill and in a slipping condition before an adjustment is made by the vehicle control system. The control system may, as a result, have limited effectiveness in terms of quickness and accuracy in compensation and achieving a smooth transition as felt by the vehicle driver.
An improved method for shifting torque between vehicle axles when encountering a hill, particularly a hill with conditions that reduce road friction, is desired.